The following patents and publications are believed to represent the current state of the art:
U.S. Pat. Nos. 6,377,238; 6,281,238; 5,767,842; 5,457,550; 5,181,181 and 4,843,568.
Foreign Patent Documents: WO 00/21024; EP 0982 676 A1; DE 298 02 435 U1; WO 01/93182 and WO 02/054169.